1. Field of Invention
The present invention relates to a timekeeping device and to a satellite signal reception method for a timekeeping device that acquires the current date and time by receiving radio signals transmitted from positioning satellites such as GPS satellites.
2. Description of Related Art
The Global Positioning System (GPS) for determining the position of a GPS receiver uses GPS satellites that circle the Earth on known orbits, and each GPS satellite has an atomic clock on board. Each GPS satellite therefore keeps the time (referred to below as the GPS time or satellite time information) with extremely high precision.
A radio-controlled timepiece that adjusts the time using time information (GPS time) from GPS satellites is taught, for example, in Japanese Unexamined Patent Appl. Pub. JP-A-H10-10251.
The signals (navigation message) from the GPS satellites are transmitted in frames and subframes synchronized to a Coarse Acquisition code (C/A code) that is reset at the beginning of the week of the GPS time. If the navigation message can be interpreted using this C/A code, the time passed since the beginning of the week in GPS time can be known from the signal from a single satellite, and the time can be set to a precision of approximately 0.1 second.
More specifically, the GPS satellites orbit at an altitude of approximately 20,000-27,000 km. It therefore takes approximately 66.6-90 ms for signals to travel from the GPS satellite to the GPS receiver. By correcting for this transmission time, the time can be synchronized to the Coordinated Universal Time (UTC) with error on the millisecond order, and a timepiece with sufficient practical precision can be achieved.
The electronic timepiece taught in Japanese Unexamined Patent Appl. Pub. JP-A-H10-10251 is a timepiece such as a table clock or a wall clock that is used in a fixed position, and assumes that signals from the GPS satellite are also received at a fixed position. Furthermore, because a GPS satellite orbits the Earth once approximately every 12 hours, which GPS satellites can be received at the present time can be determined and selectively captured by referencing the satellite history and orbit information from the last time satellite signals were received, and the satellite signals can therefore be received quickly and reliably.
When the timepiece is worn by the user, such as a wristwatch, however, signals are received while the timepiece is being worn and therefore may be moving, and signals may be received while indoors. Compared with timepieces that are used in a fixed location, there is a greater possibility that satellite signal reception by timepieces that are worn will be manually triggered by the user. It is therefore likely that the time and location where a portable timepiece such as a wristwatch executes the satellite signal reception process will be different from the previous reception process, and the previous satellite signal reception history can therefore not be used effectively.
In addition, because the amount of memory that can be provided in a portable timepiece such as a wristwatch is limited, it can be difficult to store a sufficient reception history and orbit information for each satellite, and this further limits using the reception history effectively.
As a result, a wristwatch that can receive signals from GPS satellites must typically start the satellite search from a cold start condition, that is, a condition in which there is no locally stored satellite orbit information.
This means that the GPS receiver must search indiscriminately for a GPS satellite in order to receive the time information. If the navigation data (NAV data) can be decoded from the first GPS satellite captured in this search process, the reception time can be shortened and power consumption can be reduced.
However, if the signal from the captured GPS satellite is weak, the user of the GPS receiver changes course, or the GPS satellite becomes hidden by a tall building while the signal is being received and the GPS receiver is moving, there is a strong possibility that signal reception from the captured satellite will be interrupted. It then becomes necessary to search for a GPS satellite again, thereby increasing the total reception time and increasing power consumption.